Absolute pressure transducer

ABSTRACT

Disclosed is an absolute pressure transducer including a mechanical assembly comprising an open bellows, a suspension system, and a sealed bellows arranged to yield a torque about a single sensitive axis when pressure is applied to the open bellows and an electronic circuit comprising a detector for sensing angular motion and a torquing device to restore the mechanical assembly toward its null position. The amount of electrically generated torque necessary to balance the pressure induced torque is proportional to, and a measure of, the applied pressure.

United States Patent w] 8/1971 Kistlein, 73/398C Palms 1 .lan. a, 1974[54] ABSOLUTE PRESSURE TRANSDUCER 2,51 1,094 6/1950 Barkas 73/368.7 XInventor: Je e M. as, 5 09 NE 68th 3,177,720 4/196-5 Metheny 73/393 XRedmond Wash' 98052 Primary Examiner-Donald O. Woodie] [22] Filed: Feb.17, 1971 Att0rneyLe Blanc & Shur 21 Appl. No.: 115,954

Related [1.5. Application Data [571 ABSTRACT v [63] 'cominuatiomimpan ofSer No 4 576 Jan 21 Disclosed is an absolute pressure transducerlncludmg 1970, P No- 36643377 I l a mechanical assembly comprising anopen bellows, a suspension system, and asealed bellows'arranged to 52'us. C1. 73/398 c, 73/393, 73/407 Yield a torque about a Single Sensitiveaxis when P 51 m. CI. G01l-9/i2, G011 19/04 Sure is applied to P bellowsand an electronic [58] Field of Search..........l 73/393, 407 398 C,Circuit comprising a detector for Sensing angular 7 7 308/213; 92/1, 39tion and a torquing device to restore the mechanical assembly toward itsnull position. The amount of e1ec- 5 References Cited tr'icallygenerated torque necessary to balance the UNITED STATES PATENTS pressureinduced torque is proportional to, and a measure of, the appliedpressure. 3,211,004 10/1965 Spencer 73/398 R 3,602,047

4 Claims, 12 Drawing Figures PATENTED 8W 3,783,693

SHEET 1 OF 3 F I 5 W HG. I i INVENTOR JEROME M. PAROS BY 622M @ZQLATTORNEYS.

PATENTEDJm 88974 sumenra INVENTOR JEROME M. PAROS ATTORNEYS,

MENTED 8 SHEET 3 BF 3 INVENTOR JEROME M. PAROS ATTORNEYS.

1 ABSOLUTE PRESSURE TRANSDUCER This application is a'continuation-in-part of copending application Ser. No. 4,576,filed Jan.21, 1970, now US. Pat. No. 3,664,237.

This invention relates to an improved pressure transducerincorporating'a novel pressure sensor and, more particularly, to atransducer of the force feedback type having improved accuracy,sensitivity, small size, and increased ruggedness. The pressure sensorand pressure transducer may be used in various configurations to measureabsolute pressure. The pressure sensor comprises a symmetricalarrangement of a bellows sealed at two ends, and another bellows withone sealed and one open end, both attached to a suspension system suchthat when pressure is applied to the open bellows end, a torque isproduced about the pivot point of the suspension system. In the completetransducer, a center torque to restrain the instrument at its nullposition is generated by a feedback or servo loop including detectionand torque producing means.

Previous instruments in the field of pressure sensing (which operate inthe range of from fractions of an at-' mosphere to several atmospheres)employ varying mechanical configurations of either Bourdon tubes,bellow's, diaphragms, orliquid columns. In common usage is the B'ourdontube consisting of a thin flat tube sealed at one end which acts like aspring, extending with overpressures and curling up when underpressuresare applied. The deflection produced may actuate a mechanical pointer orelectronic display. These devices are not null seeking nor force balancetype devices but must produce a deflection which is an indirect measureof the applied pressure.

Liquid column instruments include the U-tube manometer, well-typemanometer, and inclined tube manometer. These instruments depend on theheight of a liquid column to indicate the pressure head. They are notreadily adapted to recording instruments, have slow response times, andare not small, self-contained sensors.

Metal diaphragm type devices are usually used in special applications,such as fast response dynamic gages,due to the extremely smalldeflections produced unclerapplied pressure. Larger deflections areproduced, with plastic orfabric ,diaphragms; however, these are notamenable to the production of precision measuring devices.

Bellows have been employed in pressure sensors of the servo or forcebalancetype instruments. In one such instrument, a force is produced bya bellowsmechanical linkage assembly under applied pressure,

the motion sensed and the instrument kept at its null position by aforcing device whose magnitude is proportional to the unbalance force.Devices of this type are extremely sensitive to linear acceleration.

In order to overcome these and other problems, there is disclosed inassignees copending application Ser.

larly adapted for use as an absolute gage with improved temporalstability and improved accuracy.

, pensation scheme, and greater range utilization than inpreviousconstructions. It forms a small self-contained, highly precisesensor capable of operation over a large pressure range from fractionsof an atmosphere to several atmospheres. The pressure sensor may bebalanced so as to render it insensitive to linear accelerations byplacing the sensor center of gravity at the pivot point. The pivot isextremely flexible about the sensitive axis, thus making possible themeasurement of very slight changes in absolute pressure. High ridigityof the suspension system about the two other mutually perpendicular axesdecreases the sensitivity to cross axis forces and increases theinstruments ruggedness and reliability. Incorporation of thebellows-pivot arrangement into a servo or force feedback system yields adirect measurement of the applied pressure.

According to a preferred embodiment of the present invention, an openbellows-sealed .bellows arrangement, symmetrically placed about a pivotpoint,is attached at one end to a movable plate forming part of acapacitive pickoff. The electrical restoring torque which counteractsthe unbalanced pressure produced torque is generated by passage of acurrent through an annular forcer coil mounted on the moving plate whilepositioned in the field of a permanent magnet. The forcercoil isconnected through an electrical circuit to a capacitive pickoff,whichincludes the movable plate, such that the coil forms a part of thefeedback or servo loop and the current flowing in the coil necessary tomaintain the torque balance is proportional to the applied pressure.

It is, therefore, one object of the present invention to provide animproved absolute pressure transducer.

Another object of the present invention is to provide an improvedpressure sensor in the form of an open bellows-sealed bellows-suspensionsystem arrangement.

Another object of the present invention is to provide a pressure sensorwhich produces an output indicative of torque rather than displacement.

Another object of the present invention is to provide an absolutepressure sensor with a stress-free suspension system. Another object ofthe present invention is to provide an absolute pressure sensor with aunique temperature compensation scheme.

incorporating an open bellows-sealed bellows-pivot arrangement.

Another object of the present invention is to provide a servo pressuretransducer which measures torque rather than displacement and whichincorporates as part of the transducer a forcer coil and capacitivepickoff including a movable plate attached to a bellowspivotarrangement.

These and further objects and advantages of the in-.

vention will be more apparent upon reference to the followingspecification, claims, and appended drawings, wherein:

FIG. 1 shows an open bellows-pivot assembly constructed in accordancewith the present invention;

FIG. 2 illustrates the operation of the bellows-pivot assembly of FIG. 1when subjected to overpressure and underpressure;

FIG. 3 illustrates a sealed bellows-bending beam arrangement inaccordance with the present invention;

FIG. 4 shows a symmetrical construction comprising an openbellows-bending beam-sealed bellows arrangement;

FIG. 4A is a plan view of the symmetrical construction of FIG. 4;

FIG. 5 shows a symmetrical combination open bellows and sealed bellowsconstruction provided with a flexure hinge pivot of the type illustratedin FIGS. 1 and 2;

FIG. 6 shows a modified symmetrical construction with a cross springpivot;

FIG. 7 is an elevational view, with parts in section, showing an overallabsolute pressure transducer constructed in accordance with the presentinvention;

FIG. 8 is a cross section taken along line 88 of FIG.

FIG. 9- is a cross section taken along line 9-9 of FIG.

FIG. 10 shows a circuit diagram for the absolute pressure transducer ofFIG. 7; and

FIG. 11 shows a modified circuit diagram for the pressure transducer ofFIG. 7.

Referring to the drawings, FIG. 1 shows a bellowspivot assemblyconstructed in accordance with the present invention comprising a metalbellows 10 separated from a flexure hinge pivot 11 and connectedtogether by plates 12 and 13. The bellows is provided with a series ofcorrugations or convolutions 14 and is sealed at one end by an end cap15 which may be formed integral with the corrugations or joined as aseparate plug. The other end of the bellows is provided with a straighttubular section 16 terminating in an open end 17 adapted to receive'apressure fluid,.such as air, gas, liquid, or the like. The pivot is aflexure hinge formed from round or rectangular stock with a locallynecked-down portion 18. The flexure is extremely compliant aboutone'axis and flexes easily in the direction shows by double-ended arrow19 upon application of bending forces generated by the application orwithdrawal of pressure in the bellows through port 17; however, theflexure is quite stiff about the two nonsensitive mutually perpendicularaxes and is thus unresponsive to external forces along those axes.Plates l2 and 13 form the connecting assembly between the bellows andpivot, thus setting the nominal moment arm, or distance between thecenterline 20 and the bellows and centerline 21 of the flexure, overwhich the pressure induced force acts. The bellows and flexure are madefrom the same or similar materials, such as stainless steel, copper, orthe like, so as to minimize errors induced by differential temperaturegrowth. The flexure pivot and bellows are connected to the plates 12 and13 by solder, welding, brazing, or the like.

FIG. 2 shows the deflection of top plate 12 relative to the fixed bottomplate 13 when the bellows 10 is subjected to both overpressure andunderpressure. When fluid at a pressure above ambient, as indicated byarrow 23, is applied through the open end 17 of the bellows 10, theforce generated in the bellows, acting over the distance betweencenterlines 20 and 21, produces a torque which deflects the assemblyabout the localized necked-down portion 18 of flexure hinge 11. Theamount of deflection is proportional to the overpressure within thebellows. Similarly, when the bellows is evacuated to a pressure lessthan ambient, as indicated by arrow 22 in FIG. 2, then the deflection isin the opposite sense. By enclosing the bellows-pivot assembly in ahousing and then evacuating and sealing the hous ing, an absolutepressure sensor may be formed. However, under normal operating orstorage conditions, with the open port 17 exposed to the atmosphere, thesusepnsion system and bellows would be in a cocked and stressedcondition which degrades the sensor performance and stability.

FIG. 3 illustrates a sealed bellows 24 having a plug 25 which is shownwelded in and sealing the end of the bellows and containing a pinch-offtube 26 for purposes of partial or complete evacuation and sealing. Theplug and pinch-off tube can be formed integral with the bellows orjoined by welding, soldering, brazing, or the like. Sealing may beaccomplished by pinching-off a tube, or welding, soldering, etc., of asmall orifice in the plug. When the bellows is evacuated at a pressurebelow ambient, the convolutions 14 will be longitudinally compressed andthe bellows length shortened. The magnitude of the foreshortening equalsthe pressure differential between ambient and the bellows times thebellows effective area divided by the bellows longitudinal spring rate.It is important that the amount of compression remain constant for agiven pressure differential, since the sealed bellows will serve as areference in following constructions. The bellows spring rate isproportional to the metal's modulus of elasticity. Except for a fewconstant modulus materials, most metals have a large thermoelasticcoefficient (fractional change of modulus with temperature) on the orderof several hundred parts per million per degree Fahrenheit. Thetherrnoelastic coeffieient is usually negative indicating that themodulus valuedecreases with increasing temperature. Thus, in most casesa metal bellows under constant compressive pressure load willcontinually shorten as the temperature increases. This effect can benegated by partially filling the bellows interior with an amount ofinert gas, such as nitrogen or other well knwon gas, so that when thetemperature increases, the gas expands and tends to lengthen the bellowsby the same amount that the modulus decrease is foreshortening. Thebellows length and consequently forces and torques generated by thebellows are now constant with temperature variations and an excellentreference is obtained. Bellows 24 is shown attached to a flat flexiblebending beam 29 which restricts motion in all but one sensitivedirection. The sealed bellowsbending beam assembly is shown in itsneutral position at the ambient pressure at which it was assembled.However, if ambient pressure changes, an angluar deflection results, asindicated by the double ended arrow 29'.

FIG. 4 shows a symmetrical open bellows-bending beam-sealed bellowsarrangement usable as the sensitive assembly in an absolute pressuresensor. It comprises a sealed bellows construction 27, similar to thatdescribed in FIG. 3, and an open-ended bellows 28, attached to the flatflexible bending beam 29. FIG. 4A is a top view of FIG. 4 showingattachment of the bellows convolutions to the bending beam by solderjoints, such as fillet 30. Each convolution may be attached to thebending beam by soldering, welding, brazing, or the like. Referringagain to FIG. 4, bellows 29 is shown with the convolutions compressedrelative to open bellows 29. This assumes that attachment of the bellowsto the beam has been performed at about one atmosphere ambient pressureand that the sealed bellows 27 has on the order of several psi internalpressure to compensate for temperature effects on modulus. The bendingbeam readily flexes about an axis perpendicular to the plane of theillustration and passing through the center of beam 29 in FIG. 4, but isstiff in the two crossaxes directions. Thus, if undrepressure or vacuumis applied to open end 31, a clockwise torque and angular deflection isproduced. Similarly, if overpressure is applied, a counterclockwisedeflection results. The amoun'tof deflection is proportional to theapplied pressure.

FIG. 5, in which like parts bear like reference numerals, shows a sealedbellows 27 and open bellows 28 symmetrically placed about flexure hingepivot 11. Plates 12 and 13 space and hold the bellows-pivot arrangement.The flexure hinge has a locally neckeddown portion 18 which readilypermits flexing about one sensitive axis but is stiff about the twocross axes. The sensor is assembled normally at one atmosphere ambientpressure so that plates 12 and 13 are parallel and no angular deflectionof flexure pivot 11 is produced under normal storage conditions of oneatmosphere pressure being applied to open end 31 and the suspensionsystem is unstressed. Upon application of pressure under one atmosphere,a clockwise deflection is produced, and when overpressure is applied, acounterclockwise deflection results. If twoopen ended bellows had beenassembled as shown and-then one bellows evacuated and sealed, the plates12 and 13 would be fully deflected and the suspension stressed.Furthermore, extra range is obtained with the pre-sealed bellowsassembly since the plates are neutral at a pressure of one atmosphereand deflect equal amounts clockwise and counterclockwise whenrespectively vacuum or two atmospheres absolute pressure is appliedthrough open bellows end 31. A method of making the sensor insensitiveto acceleration forces is to place a balance weight 32 of the propersize and position such that the center of gravity of the movable system,i.e.,

weight 32, coincide with the pivot point of the flexure. An alternatemethod is to physically move the flexure hinge pivot point untilcoincidence with the center of gravity is obtained. Thus, with thecenter of gravity balanced to be at the pivot point, transverseaccelerations .produce no deflections and have no effect on the outputof the instrument.

FIG. 6 shows a modified suspension system employing crossed flexurestrips 33 and 34 which permitrotation about a single axis through point35 as indicated by arrow 36 when over or underpressure is applied toopen bellows 28. v

FIG. 7 is a view with parts in section showing an overall absolutepressure transducer constructed .in accordance with the presentinvention. The sensor is of the open bellows-flexure hinge-sealedbellows construction although the bending beam and flexure striparrangements described previously are also applicable. Open bellows 40and sealed bellows 41 are symmetrically placed about flexure pivot 42.The straight, tubular, open ended bellows section 46, sealed bellows end47, as well as end 53 of flexure hinge 42, are soldered or braced intobase plate 43. Supply tube 44, which is also brazed or soldered intobase plate 43, provides the inlet port for the pressure fluid whichcommunicates with bellows through tubular section 46. The closed ends 55and 56 of bellows 40 and 41 and end 54 of flexure hinge 42 are solderedor brazed to end plate 52 which is integral with sides 57 and 58 to thelaterally extending annular flange 61. Flange 61 serves as the movableplate in a capacitive pickoff, as well as the support for annular forcercoil 62. Fixed capacitance plates 74 and 75 are electrically insulatedfrom base plate 42 by circular segments 72 and 73 which, together withmovable plate 61, form a differential capacitive pickoff for the servotransducer. The forcer coil 62 surrounds a permanent'magnet 65 which islinearly polarized in the plate of the paper as shown in FIG. 7. Anannular magnetic return ring is used to completethe magnetic circuit; Asshown, the flux lines go from the north pole of the magnet through theadjacent side of coil 62 around both' halves of soft iron ring 76 and byway of the opposite side of coil 62 tothe south pole of the magnet. Anonmagnetic spacer 96 separates the magnet 65 and annular return ring 70from the housing 93.

bellows 27 and 28, flexure 11, plate 12, and balance FIG. 8, which is asectional view perpendicular to that of FIG. 7, more clearly shows theshapes and relationships described previously. The bellows end caps 55and 56 and flexure hinge end 54 are attached to end plate 52 which isintegral with the moving capacitance plate and serves as a support forforcer coil 62. Not attached to the moving system but positioned betweenend plate 52 and coil 62 is permanent magnet 65. The magnetic returnring is completed through annular section 70.

FIG. 9 is a sectional view along line 9-9 of FIG. 7 showing receptacles48, 49, and 56, into which the two bellows ends and flexure hinge arerespectively soldered or brazed into base plate 4-3. Fixed capacitanceplates 74 and 75 are shown as well as electrical feedthrough terminals36, 31, 32, and 83. Fixed capacitance plates 74 and 75 are electricallyattached to feedthrough terminals and 81, respectively. Flexibleelectrical leads from the forcer coil are brought from the moving systemto terminals 82 an 83. Referring to FIG. 7, movable capacitance plate 61is integral with end plate 52 and connected through the bellows andflexure hinge to grounded base plate 43. The grounded movable plate isconnected to the electronic circuitry 56 by means of flexible lead 58 tometal spacer 87. The

forcer coil leads 39 and and fixed capacitance plate leads 91 and 92 areconnected to the electronic circuitry 86 from the opposite ends of thefeedthrough terminals 52, 53, 811, and 81, shown in FIG. 9. FIG. 7

also shows electrical connector 95 through which power is supplied tothe servo electronics and the output signal exits. Housing 94, throughwhich supply tube 44 passes and connector 95 is mounted, also acts as acover for the electrical assembly.

The moving system, consisting of the bellows, pivot, moving plate, andforcer coil,- is balanced about an axis through pivot point 60 such thatthe assembly is insensitive to acceleration forces. Balancing may beachieved by addition of weights to'the movable plate or by movement offlexure hinge 42.

Operation of the sensor will be described with reference to FIG. 7.Application of an overpressure to bellows 40 through supply tube 44generates a force which is transmitted through end plate 52 to provide acounterclockwise torque and resultant deflection about pivot point 60.The angular deflection brings the moving capacitance plate 61 intocloser proximity to fixedplate 74 while the opposite side of plate 61moves away from fixed plate 75. The capacitance change is detected andan electrical current is sent through coil 62 in the field of permanentmagnet 65 to produce a counteracting torque which restores the assemblyto its null position. The current is thus proportional to and a measureof the applied pressure. A clockwise torque is produced whenunderpressure or vacuum is applied to supply tube 44. Movable plate 61moves toward fixed plate 75, the capacitance change is-detected, and thefeedback current restores the sensor toward its null position.

FIG. is a simplified circuit diagram of the pressure sensor. The circuitcomprises an AC source or generator 106 operating,by way of exampleonly, at a frequency of about 6 megahertz. One side of source 106 isgrounded as at 108 and connected across it is the primary 110 of anisolating transformer 112. Transformer 112 is provided with twosecondary windings 114 and 116, one side of the transformer secondariesbeing connected through capacitors 118 and 120 to the inputs of adifferential amplifier 122. The output of amplifier 122 is proportionalto the difference between the signals at its two inputs and theamplifier develops an output signal causing current to flow through theforcer coil 62, the amount of this current being indicated by a meter124. The other sides of the transformer secondaries 114 and 116 areconnected to the stationary plates 74 and 75 which, together withgrounded moving plate 61, form a differential capacitor.

When plate 61 is in its center position, the signals on the secondarywindings of the transformer are balanced and no signal is developed atthe output of amplifier 122. However, as movable capacitor plate 61tends to move with a deflection of the bellows-pivot assembly underpressure, an unbalance in the signal between the two inputs of theamplifier 122 occurs and a signal is developed at the output of theamplifier in the form of a current through forcer coil 62 whichgenerates a counter-torque tending to restore capacitor plate 61 to itsinitial position. The amount of current flowing through forcer coil 62to produce a torque balance condition in the transducer is proportionalto the pressure differential in the two bellows and the amount of thiscurrent is indicated by meter 124 which gives a direct indication of thedifferential pressure.

FIG. 11 shows a modified circuit for the transducer of FIG. 1. In FIG.11, like parts bear like reference numeral and the circuit of FIG. 11 issimilar to the circuit of FIG. 10 described above with the exceptionthat the moving plate 61 is not grounded but instead is returned to aterminal 126, Le, is returned to the midpoint between a pair ofresistors 128 and 130 connected across the differential capacitor.Resistors 128 and 130 are provided in the circuit of'FIG. 11 to take theDC leakage off the plates of the differential capacitor.

It is apparent from the previous discussion that the present inventionprovides an improved pressure sensing element and an improved absolutepressure transducer. The unit is small, self-contained, highly accurate,and capable of operation over a large pressure range from smallfractions of an atmosphere to several atmospheres. The instrument may beeasily balanced to make it insensitive to acceleration forces by properplacement of balance weights.

Movement of the pivot point can be used to place the center of gravityat the pivot point, thus balancing the device and making it insensitiveto linear forces. High rigidity of the pivots in the cross axesdirections improves the sensor's ruggedness. Incorporation ofthe sensingelement into a servo loop yields a direct measurement of the appliedpressure. The pivots are extremely flexible about the sensitive axis,thus providing negligible mechanical restraint and making thecounteracting torque generated by the electrical current a directmeasure of the applied pressure. By filling a bellows with the properamount of gas prior to sealing, a temperature insensitive pressurereference is obtained which obviates the use ofa select few materials.Assembling of an open bellows-pivot-sealed bellows arrangement at apressure corresponding to normal storage or operating conditions yieldsa stress free suspension system and a more stable instrument. Whenincorporated in a servo loop, minimum restoring current is required atthis assembly pressure. Excursions of the movable assembly from its nullposition are possible in both angular senses, thus increasing theeffective range of the sensor.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:

1. A servo rebalance type pressure sensor comprising a first bellowswith one open end and one closed end, a second sealed and at leastpartially pre-evacuated bellows, flexible pivot means, said first andsecond bellows being mounted on opposite sides of said pivot means, saidsecond bellows being evacuated, partially compressed and foreshortenedprior to mounting, means interconnecting said first and second bellowsand said pivot means such that said pivot means is in an undeflectedposition under ambient pressure conditions, a torque balance servo loopincluding means to sense deflection of said pivot means, and meanscoupling said loop to both said bellows whereby said pivot means istorqued toward said undeflected position by said servo loop, wherebyalthough said second bellows is at least partially evacuated forpressure measurement, said pivot is unstressed under ambient pressureconditions and whereby said torque balance servo loop is nulled underambient pressure conditions thereby to increase shortens said secondbellows with temperature increases.

said pickup tending to restore said pivot means to said undeflec'tedposition. v

4. Apparatus according to claim 3 including a meter coupled to saidforcer coil for indicating the current flow through said coil.

I UNITED EY IY'UEI J QS PA'IFIKT OFFICE 'CEPLTIFKCATE Q1" Q0 RK'ECTIQNPatent No. 3,783,693 Dated January 8 1974 Inventorfls) Jerome M. ParosIt is certified that error appears in the above-identified parent andthat said Letters Patent are hereby corrected as shown below:

Col. 5, line 14, "undrepressure" should read --underpressu 1:'e---..

Col. 6, line 19, "plate" should read --plane--; line 46, "an 83" shouldread --and 83 Col. 7, line 55, "FIG. 1" should read --FIG. 7-.

Col. 9, lines 3-4, claim 3, "said both" should read 5 --both said-5;line 6, "restoring" should read --a restoring--.

Signed and sealed this 16th day of July 1971 (SEAL) Attest:

MCCOY M. GIBSON, JR. c. MARSAHLL 'DANN I Attesting Officer Commissionerof.Patents

1. A servo rebalance type pressure sensor comprising a first bellowswith one open end and one closed end, a second sealed and at leastpartially pre-evacuated bellows, flexible pivot means, said first andsecond bellows being mounted on opposite sides of said pivot means, saidsecond bellows being evacuated, partially compressed and foreshortenedprior to mounting, means interconnecting said first and second bellowsand said pivot means such that said pivot means is in an undeflectedposition under ambient pressure conditions, a torque balance servo loopincluding means to sense deflection of said pivot means, and meanscoupling said loop to both said bellows whereby said pivot means istorqued toward said undeflected position by said servo loop, wherebyalthough said Second bellows is at least partially evacuated forpressure measurement, said pivot is unstressed under ambient pressureconditions and whereby said torque balance servo loop is nulled underambient pressure conditions thereby to increase the range of saidsensor.
 2. Apparatus according to claim 1 wherein said second bellowscontains a temperature compensating gas below atmospheric pressure andhaving a gas expansion characteristic which tends to lengthen saidsecond bellows by an amount that the modulus decrease foreshortens saidsecond bellows with temperature increases.
 3. Apparatus according toclaim 1 wherein said torque balance servo loop includes a forcer coiland capacitive pickup having a portion movable with said both bellows,and a permanent magnet adjacent said forcer coil whereby current throughsaid forcer coil generates restoring torque in response to variations ofsaid pickup tending to restore said pivot means to said undeflectedposition.
 4. Apparatus according to claim 3 including a meter coupled tosaid forcer coil for indicating the current flow through said coil.